Storage of fuel cell energy during startup and shutdown

ABSTRACT

During startup or shutdown of a fuel cell power plant, the electric energy generated by consumption of reactants is extracted by a storage control ( 200 ) in response to a controller ( 185 ) as current applied to an energy storage system  201  (a battery). In a boost embodiment, an inductor ( 205 ) and a diode ( 209 ) connect one terminal ( 156 ) of the stack ( 151 ) of the battery. An electronic switch connects the juncture of the inductor and the diode to both the other terminal ( 155 ) of the stack and the battery. The switch is alternately gated on and off by a signal ( 212 ) from a controller ( 185 ) until sufficient energy is transferred from the stack to the battery. In a buck environment, the switch and the inductor ( 205 ) connect one terminal ( 156 ) of the stack to the battery. A diode connects the juncture of the switch with the inductor to the other terminal ( 155 ) of the fuel cell stack and the battery.

TECHNICAL FIELD

This invention relates to storing electrical energy extracted from afuel cell stack during startup and/or shutdown, as an alternative todissipating that energy in an auxiliary load or other resistive voltagelimiting devices.

BACKGROUND ART

It has been known that corrosion of amorphous carbon catalyst supportsand metal catalyst, which occurs during startup and shutdown of polymerelectrolyte membrane (PEM) fuel cells, results in a permanent decay offuel cell performance. It has also been known that the corrosion is dueto a reverse current situation in which the cathode potential may bewell in excess of one volt higher than the potential of a standardhydrogen electrode. It is believed that this is caused by both hydrogenand air being present at different locations within the anode flowfield. During a shutdown period, unless an inert gas purge is used, airwill slowly, uniformly fill both the anode and cathode flow fields ofthe fuel cell. During startup, hydrogen is fed to the anode flow fieldwhich results in the inlet to the anode flow field being primarilyhydrogen while the exit of the anode flow field is primarily air. Anelectrochemical reaction occurs between the fuel rich zone in the anodeflow field and the oxygen rich zone in the anode flow field that causesthe potential of the anode in the oxygen rich zone to increase to theair open-circuit potential. This in turn raises the potential of thecathode, opposite to the air rich zone on the anode, to a potential of1.4-1.8 volts versus a standard hydrogen electrode. This potentialcauses the carbon based catalyst support to corrode and results indecreased cell performance.

In copending U.S. patent application Ser. No. 09/742,481, filed Dec. 20,2000, it is shown that as the fresh hydrogen-containing fuel flowsthrough the anode flow field upon startup, to displace the air therein,the corrosion of the platinum catalyst and catalyst support occurs asthe hydrogen/air interface moves through the anode flow field. Theextent of corrosion is mitigated by rapidly purging the air withhydrogen during startup of the fuel cell. In a similar fashion, it isknown that as purge air is passed through the anode upon shut-down,there is a hydrogen/oxygen interaction, which creates a potential safetyhazard and may cause undesirably large voltage excursions in the cells,as described in copending U.S. patent application Ser. No. 09/742,497,filed Dec. 20, 2000.

In automotive applications, that may experience 50,000-100,000startup/shutdown cycles, this results in catastrophic performance loss.Heretofore, solutions to this problem include stabilizing the fuel cellstack by purging the anode flow fields with an inert gas, such asnitrogen, and maintaining an auxiliary load across the fuel cell stackduring the shutdown and startup processes.

In automotive applications, the availability of an inert gas, and theapparatus to employ it for purging will be prohibitively complex andexpensive. The use of an auxiliary load requires dissipation of the heatgenerated thereby, which may typically occur in a reservoir of a watercirculation system or a coolant system, or may occur with air cooling.

Referring now to FIG. 1, a vehicle 150 includes a fuel cell stack 151comprising a plurality of contiguous fuel cells, only one fuel cell 12being shown in FIG. 1. The electrical output at the positive andnegative terminals of the fuel cell stack 151 is connected by a pair oflines 155, 156 through a switch 158 to a vehicle propulsion system 159.The output is also connected through a switch 160 to an auxiliary load161 in a reservoir 164 of a water circulation system, the reservoirhaving a vent 165. The water circulation system may include a trim valve166, water passages, such as those within water transport plates 84, aradiator and fan 168, 169 which is selectively operable to cool watercirculating in the system, and a water pump 170. Ambient air at an inlet173 is provided by a pump, such as a blower 174, to the oxidant reactantgas flow fields of the cathode 19, and thence through a pressureregulating valve 175 to exhaust 176. Hydrogen is supplied from a source179 through a flow regulating valve 180 to the fuel reactant gas flowfields of the anode 17, and thence through a pressure regulating valve181 to exhaust 182. A fuel recycle loop includes a pump 183.

A controller 185 responds to load current determined by a currentdetector 186 as well as to the voltage across the lines 155, 156; it mayalso have temperature of the stack provided on a line 187. Thecontroller, in turn, can control the valve 180 over a line 190 as wellas controlling the other valves, the switches 158, 160 and the pumps174, 170, as shown in FIG. 1.

The controller 185 responds to start and speed control signals from thevehicle propulsion system 159 on lines 193 and 194, which will indicatewhen the fuel cell should commence operation, and the amount of powerbeing demanded by the vehicle propulsion system. Whenever a startupsignal is sent from the vehicle propulsion system 159 over the line 193to the controller 185, signals from the controller will cause the valves180, 181 and the pump 183 to be operated appropriately so as to providefuel reactant gas to the flow fields of the anode 17, and the valve 175and pump 174 will be operated appropriately to provide ambient air tothe flow fields of the cathode 19.

When fuel and air of sufficient quantity have been provided uniformly tothe cells, open circuit voltage will be detected on the lines 155, 156by the controller 185. At that time, the controller may close switch 160so as to connect the fuel cell stack 151 to the auxiliary load 161 inthe reservoir 164 and may also close the switch 158 so as to connect thefuel cell stack 151 to the vehicle propulsion system 159 at the sametime, or later.

Whenever a shutdown signal is received from the vehicle propulsionsystem 159, the switch 160 will be closed so as to connect the auxiliaryload 161, as the switch 158 is open so as to disconnect the vehicle fromthe fuel cell power plant.

DISCLOSURE OF INVENTION

Objects of the invention include: eliminating the need for an auxiliaryload or other resistive voltage limiting device to control corrosion andperformance decay in startup and shutdown of fuel cell stacks;conserving energy in a fuel cell power plant; controlling fuel cellreactions during startup and shutdown in a manner closely related to thethen-present conditions; and making otherwise wasted energy availablefor use in a fuel cell power plant.

According to the present invention, during startup or shutdown of a fuelcell stack, the spurious energy generated by the consumption ofreactants therein is extracted in the form of electrical energy andstored in an energy storage device associated with the fuel cell powerplant. In a boost configuration, useful when the voltage of the fuelcell stack is lower than the voltage at which it is desired to storeenergy in the energy storage device, an electronic switch causes currentto build up in an inductor, and when the switch is gated off, thecurrent continues to flow through a unilaterally conducting device intothe energy storage system. In a buck configuration which is useful whenthe voltage of the stack is greater than the voltage at which energy isto be stored in the energy storage system, one electrical outputterminal of the stack is connected through an electronic switch and aninductor to one side of the energy storage device, a second electricaloutput terminal of the fuel cell stack being connected to the other sideof the energy storage system; a unilaterally conducting device extendsfrom the second terminal to the juncture of the electronic switch andthe inductor; when startup or shutdown is to occur, the electronicswitch is first gated on causing current to flow directly through theinductor into the energy storage system; then the electronic switch isgated off and current continues to flow through the unilaterallyconductive device and the inductor into the energy storage system.

During startup, this process is repeated for a given period of time, oruntil the DC current stabilizes at a specified level, or until aspecific amount of energy is transferred. During shutdown, this processis repeated until the voltage decays below a specified level (the energyin the fuel cell is dissipated).

In accordance further with the invention, in an electric vehicle poweredby a fuel cell power plant, the energy storage system is a battery whichis utilized for regenerative braking by the electric vehicle. The energystorage system is generally only charged to about 80% of its capacity toallow for regenerative braking, and to permit storing the energy of thefuel cell stack as a consequence of startup or shutdown.

Configurations other than those described above (and to be described inmore detail hereinafter) may be utilized to practice the invention. Suchconfigurations may include the use of isolation transformers, andvarious power electronics topologies, such as buck-boost, push-pull,forward, and flyback. Various switching devices may also be utilized topractice the invention.

The invention avoids the need for dissipation of heat, avoids the needfor auxiliary loads or other voltage limiting devices, and is easilyprogrammable to suit current operating conditions, which are differentduring startup than they are from shutdown, and to suit otheroperational variables.

Other objects, features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof exemplary embodiments thereof, as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a fuel cell power plant known tothe prior art, utilizing an auxiliary load for startup and shutdown.

FIG. 2 is a schematic block diagram of a fuel cell power plant thatstores the energy of a fuel cell stack in an energy storage system,during startup and shutdown, in accordance with the invention.

FIGS. 3 and 4 are schematic diagrams of a boost configuration storagecontrol and a buck configuration storage control, according to theinvention.

FIGS. 5 and 6 are graphs of power versus time.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring to FIG. 2, an auxiliary load (161, FIG. 1) is not utilized.Instead, a storage control 200 extracts the energy stored in the fuelcell stack, during startup or shutdown, and applies it to an energystorage system 201, which in the present embodiment is the battery of anelectric vehicle which is powered by the vehicle propulsion system 159.In other embodiments, the energy storage system 201 may be some otherbattery, it may be a capacitor, or it may be some other electricalstorage device.

The storage control 200 may take the form shown in FIG. 3, which is aboost configuration useful when the voltage output of the stack is lowerthan the voltage at which energy is to be stored in the energy storagesystem. In FIG. 3, an inductor 205 is connected in series with anelectronic switch 206, which may be an insulated gate bipolartransistor, as shown, or any other suitable electronic switch, betweenthe electric output terminals 155, 156 of the fuel cell stack 151.

The output of the storage control on a line 208 is taken from thejuncture of the inductor and the switch through a unilaterallyconducting device such as a diode 209. In order to transfer energy fromthe cell stack 151 when the output voltage thereof is less than thevoltage at which the energy is to be stored in the ESS, the switch 206is first gated on by a signal on a line 212 from the controller 185(FIG. 2), so a current builds up in the inductor 205. After a time, theswitch 206 is gated off and the current in the inductor will continue toflow through the diode 209 and the output line 208 into the energystorage system 201 (FIG. 2), which may be a battery 213. The currentthrough the output line 208 (and the other terminal 155 of the fuel cellstack) is stored in the energy storage system 201. When energy leavesthe fuel cell stack, in the form of current, the voltage in the fuelcell stack will decrease. This process is continued until the desiredenergy has been extracted from the fuel cell stack.

As an example of the energy relationship, FIG. 5 illustrates that theamount of energy to be transferred from the fuel cell stack can becalculated by plotting the output of the fuel cell stack, thetransferred energy being represented by the area of the curve. Theamount of energy is represented by the integrated power versus time thatis generated by the fuel cell stack during a start or stop transition.In this example, the energy, E, is equal to 15 kiloJoules and the poweris dissipated in three seconds.

According to the invention, the energy is not taken out uniformly, ascan be seen in FIG. 5. Instead, the transfer of power quickly reaches amaximum, and then decreases with respect to time. In the configurationsdescribed with respect to FIGS. 3 and 4, the energy is transferred inincrements, as the switch 206 is gated on and off, as is illustrated inFIG. 6.

In FIG. 4, the switch 206 is in series with the inductor 205 between oneelectrical terminal 156 of the fuel cell stack and the energy storagesystem (201, FIG. 1). The diode 209 is connected from the otherelectrical terminal 155 of the fuel cell stack to the juncture betweenthe inductor 205 and the switch 206. In the buck configuration of FIG.4, which is used when the voltage of the fuel cell stack is greater thanthe voltage at which energy is to be stored in the energy storagesystem, the switch 206 is gated on by a signal on the line 212 causing acurrent to flow from the terminal 156 through the inductor 205 and intothe energy storage system over the output line 208. Then, the switch 206is gated off, at which time current will flow through the diode 209 andthe inductor 205 over the output line 208 to the energy storage system201, which in this instance is illustrated as a capacitor 215. Thecurrent flow through the switch 206 and inductor 205 causes the voltageof the fuel cell stack to decrease. The switching process is repeateduntil the desired energy has been extracted from the fuel cell stack.

In the configurations of FIGS. 3 and 4, control over the switching ofelectronic switch 206 by the signal on the line 212 allows use of theinvention both for startup and for shutdown, wherein the energyrequirements may differ between startup and shutdown. The sizing of thecomponents 205, 206, 209, will be determined to carry the maximumcurrent required for startup/shutdown.

Other configurations, particularly switching configurations may beutilized, including use of an isolation transformer which could step thevoltage up or down, in dependence on the system in which the inventionis used, the transformed current then rectified for storage in acapacitor or a battery, or other suitable storage system. In thisembodiment, the storage system is electrical, but other storage systemsmay be utilized, including mechanical systems, such as fly wheels.

All of the aforementioned patent applications are incorporated herein byreference.

Thus, although the invention has been shown and described with respectto exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made therein and thereto, withoutdeparting from the spirit and scope of the invention.

1. A fuel cell power plant adapted to store energy which is removed fromthe associated fuel cell stack during transition from being notoperating to operating, and vice versa, comprising: a controllerinterconnected with said fuel cell stack and responsive to signalsreceived by said controller to cause said fuel cell stack to start upand to cause said fuel cell stack to shut down; an energy storage systemassociated with said fuel cell power plant, said energy storage systemresponsive to electrical output provided thereto to store correspondingenergy; and storage control means operable by said controller, during atransition selected from (a) startup of said fuel cell power plant or(b) shutdown of said fuel cell power plant, to extract, in the form ofelectrical output, energy generated by said fuel cell stack, saidelectrical output being provided to said energy storage device, therebylimiting the maximum average voltage in the fuel cells of said fuel cellstack during said transition.
 2. A power plant according to claim 1wherein: said storage control means comprises an inductor in series witha unilaterally conducting device extending from one electric output ofsaid fuel cell stack to one input of said energy storage system and anelectronic switch connected from the juncture of said inductor with saidunilaterally conducting device to both a second electric output terminalof said fuel cell stack and a second terminal of said energy storagesystem; and said electronic switch is gated on and off by a signal fromsaid controller.
 3. A power plant according to claim 1 wherein: saidstorage control means comprises an inductor in series with aunilaterally conducting device extending from one electric output ofsaid fuel cell stack to one input of said energy storage system and anelectronic switch connected from the juncture of said inductor with saidunilaterally conducting device to a second electric output terminal ofsaid fuel cell stack; and said electronic switch is cyclically gated onand off by a signal from said controller.
 4. A power plant according toclaim 1 wherein: said energy storage system comprises an electricbattery.
 5. A power plant according to claim 4 wherein: said electricbattery is disposed on a vehicle powered by said power plant.
 6. A powerplant according to claim 1 wherein: said energy storage system comprisesa capacitor.